Conventional colour photographic silver halide materials are processed by a process which includes a colour development step. In this step silver halide is reduced to metallic silver in the light-exposed areas and the oxidized colour developer formed in this reaction then couples with a colour coupler and forms image dye. The amount of dye produced is proportional to the amount of silver halide reduced to metallic silver.
Redox amplification processes have been described, for example in British Patent Specifications GB-A-1268126, GB-A-1399481, GB-A-1403418 and GB-A-1560572. In such processes colour materials are developed to produce a silver image (which may contain only small amounts of silver) and then treated with a redox amplifying solution to form a dye image. The redox amplifying solution contains a reducing agent, for example a colour developing agent, and an oxidizing agent which is more powerful than silver halide and which will oxidize the colour developing agent in the presence of the silver image which acts as a catalyst. Oxidized colour developer reacts with a colour coupler (usually contained in the photographic material) to form image dye. The amount of dye formed depends on the time of treatment or the availability of colour coupler rather than the amount of silver in the image as is the case in conventional colour development processes. Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide, cobalt (III) complexes including cobalt hexammine complexes, and periodares. Mixtures of such compounds can also be used.
Since the amplifying solution contains both an oxidizing agent and a reducing agent it is inherently unstable. That is to say unlike a conventional colour developer solution, amplifier solutions will deteriorate in less than an hour even if left in a sealed container. The best reproducibility for such a process has been obtained by using a "one shot" system, where the oxidant is added to the developer and the solution mixed and used immediately (or after a short built in delay) and then discarded. This leads to the maximum solution usage possible with maximum effluent and maximum chemical costs. As a result the whole system is unattractive especially for a minilab environment where minimum effluent is required. It is believed that it is these shortcomings that have inhibited commercial use of this process.
Japanese Specification 64/44938 appears to describe such a system in which a silver chloride colour material is processed in a low volume of a single-bath amplifier solution. The processes described therein however fall short of what is required in the fully commercial environment for exactly the reasons given above.
Published International Patent Application WO-A-91/12567 (which corresponds to British Patent Application No. 9003282.2) describes a method and apparatus for photographic processing in which a minimum amount of processing solution can be used in a processing tank which is thin and has a low volume. In order to overcome the inherent deterioration problem due to the instability of the processing solutions used, the method and apparatus described result in the need for high recirculation and/or replenishment rates. However, problems associated with non-uniform processing of the photographic material may be encountered due to local differences in the concentration of the processing solution. Agitation of the processing solution appears to provide one solution to this problem.